Power source for electric arc welding

ABSTRACT

A power source for an electric arc welding process, wherein the power source comprises an input stage having an AC input and a first DC output signal; a second stage in the form of an unregulated DC to DC converter having an input connected to the first DC output signal and an output in the form of a second DC output signal electrically isolated from the first DC output signal and with a magnitude of a given ratio to the first DC output signal; and, a third stage to convert the second DC output signal to a welding output for the welding process.

The invention relates to the field of electric arc welding and moreparticularly to a power source for such welding and the methodimplemented by use of the novel power source.

INCORPORATION BY REFERENCE AND BACKGROUND OF INVENTION

Electric arc welding involves the passing of an AC or DC current betweena metal electrode and a workpiece where the metal electrode is normallya cored metal wire or solid metal wire. A power source is used to createa given current pattern and/or polarity between the advancing electrodewire and workpiece so that the arc will melt the end of the advancingwelding wire and deposit the molten metal on the workpiece. Althoughvarious converter technologies are used for power sources, the mosteffective is an inverter based power source where a switching networkincludes switches operated at high frequency to create the desiredwaveform or current level for the welding process. An inverter typepower source is discussed in Blankenship U.S. Pat. No. 5,278,390 wherethe inverter is operated in accordance with the preferred embodiment ofthe present invention. This preferred operating procedure involves“waveform control technology” pioneered by The Lincoln Electric Companyof Cleveland, Ohio where the actual waveform is generated by a series ofshort pulses created at a frequency generally above 18 kHz and the groupof short pulses has a profile controlled by a waveform generator. Thiswell known type of inverter control technique is used in the preferredembodiment of the present invention and need not be described in moredetail. In accordance with standard power source technology, the inputsignal to the inverter stage of the power source is rectified currentfrom a sine wave power supply. An appropriate power factor correctingconverter is common practice and is either a part of the inverterswitching network itself, as shown in Kooken U.S. Pat. No. 5,991,169, oris located before the inverter stage, as shown in Church U.S. Pat. No.6,177,645. Indeed, a power source with a power factor correctingconverter or stage has been known in the welding art for many years.Another power source employing an input power factor correctingconverter in the form of a boost converter is shown in Church U.S. Pat.No. 6,504,132. The two patents by Church and the patent by Kooken areincorporated by reference herein as background information andtechnology to which the present invention relates. In both Kooken U.S.Pat. No. 5,991,169 and Church U.S. Pat. No. 6,504,132 the actual weldingcurrent is regulated by an output chopper or buck converter andisolation is obtained by a transformer either in the output of theinverter stage or in the output of the input boost converter. Thesevarious topologies for power sources are common knowledge in arc weldingtechnology. In these prior art patents, the actual welding current,voltage or power is regulated in or before the output stage of the powersource, which output stage is either an inverter or a chopper. Neitherthe inverter, nor the chopper is unregulated to produce a fixed, lowervoltage DC bus for driving a regulated welding stage as anticipated bythe present invention.

Isolation of the welding operation is a characteristic of most powersupplies for welding. The term “welding” includes “plasma cutting.” InVogel U.S. Pat. No. 5,991,180, a preregulator using a boost converter isdirected to a converter which is disclosed as a chopper having an outputisolation transformer located after welding regulation and directlydriving the welding operation. In this power source, the chopper networkis controlled to create the desired regulated output welding current andisolation is provided in the output stage. In a like manner, ThommesU.S. Pat. No. 5,601,741 discloses a boost converter for driving a pulsewidth modulated controlled inverter providing the regulated outputsignal to the actual welding operation. In both Vogel and Thommes, thesecond stage is regulated to direct the power factor controlled currentfrom a preregulator into a welding operation. Welding regulation is inthe second stage and is normally driven by a pulse width modulatorcontrol circuit. Both Vogel and Thommes are incorporated by referenceherein as background technology. In Moriguchi U.S. Pat. No. 6,278,080 aninverter type power source is regulated to control the desired weldingcurrent. Isolation is obtained by a transformer between the controlledsecond stage inverter and the welding output which is disclosed as a DCwelding operation. A similar power source is shown in Moriguchi U.S.Pat. No. 5,926,381 and Moriguchi U.S. Pat. No. 6,069,811 wherein theisolation of the control current from the inverter stage is at theoutput of the inverter and directly drives the welding operation.Moriguchi U.S. Pat. No. 5,926,381 discloses the common arrangement forusing the voltage at the output of the first stage boost converter toprovide the controller voltage for either the regulated inverter stageor the boost converter itself. The three Moriguchi patents areincorporated by reference herein as background information showing theprior art power source where a regulated inverter is driven by an inputboost converter or a DC output of a rectifier to produce a controlledwelding current directed to an output transformer used for isolation.The secondary AC signal of the isolation transformer is directly usedfor the welding operation. There is no third stage topology as used inthe novel power source of the invention.

Turning now to non-welding technology, an aspect of the invention is theuse of a synchronous rectifier device at the output of a DC/DC secondstage converter. Synchronous rectifiers are common practice and one suchrectifier is illustrated in Boylan U.S. Pat. No. 6,618,274. Calkin U.S.Pat. No. 3,737,755, discloses a DC/DC converter for low power use wherea fixed regulated current is directed to a non-regulated inverter toprovide a non variable output DC signal. Any control of thenon-regulated inverter is at the input side of the inverter so that theinput DC signal is the only parameter that can be regulated to controlthe fixed output DC signal of the inverter. This is a topography thatrequires a control of the signal to the inverter so that the inverterprovides a controlled fixed output signal. This is a different conceptthan anticipated by use of the present invention; however, thenon-welding general background technology in Boylan and Calkin isincorporated by reference herein to show a synchronous rectifier and aversion of a non-regulated inverter where any regulation is performedbefore the inverter by controlling the level of the input DC signal.Neither of these patents relate to a power source for welding and areonly incorporated by reference as general technical concepts, such assynchronous rectifier devices and unregulated inverters. A non-weldingtwo stage AC to DC converter is shown in Smolenski U.S. Pat. No.5,019,952 for imparting minimum harmonic distortion to the currentflowing into the converter. The load is not variable and does notrequire regulation as demanded in a welding operation. This patent isincorporated by reference to show general technology not related in anyway to the demands of a power source for electric arc welding.

These patents constitute the background of the invention relating to apower source that must be regulated by a welding operation where suchregulation is by a feedback loop of average current, average voltage,and power of the actual welding operation. Fixed load power sources arenot relevant to the invention, except as general technical information.

In the past, an inverter in a power source outputted a welding currentregulated by a parameter in the welding operation, such as current,voltage or power. This inverter was normally controlled by a pulse widthmodulator wherein the duty cycle of the switches operated at highfrequency was controlled by the feedback from the welding operation sothat the duty cycle was adjusted in a range substantially less than100%. This type of PWM controlled inverter is referred to as a regulatedsingle stage inverter. Such inverter formed the output of the powersource and was the last stage of the power source. Lower duty cyclesresulted in higher primary currents and more losses. The efficiency ofthe inverter varied according to the duty cycle adjustment caused by therequirement of regulating the output of the single stage inverter tocreate an output signal suitable for welding. Using a power source wherethe final stage is a regulated single stage inverter resulted in heatlosses, lower efficiency, high cost and increased component size. Forthese reasons, some welding source manufacturers have marketed powersources as being better than an inverter power source because they donot use inverters with the resulting high cost and other difficulties.An inverter stage which had the dual function of isolating the outputand regulating the current for the purposes of creating a currentsuitable for welding was to be avoided. See Hoverson U.S. Pat. No.6,723,957, incorporated by reference herein as background.

THE PRESENT INVENTION

The present invention relates to a power source for electric arc welding(plasma cutting) wherein the inverter of the power source is a secondstage as in the past, but is unregulated so that a third stage can beadded to provide the actual regulation for creating a current suitablefor welding. By using this three stage concept, the inverter can operateat a very high frequency of switching whereas the output third stage canbe a chopper operated at a lower frequency of switching. Consequently,the switching frequency is optimized by the function performed by thestage as opposed to the need for using high frequency in a pulse widthmodulated inverter stage used for actual regulation of the outputwelding current. Furthermore, the isolated, fixed DC voltage to theregulated third stage can be substantially lower than the DC voltagefrom the input converter stage and much higher than the actual weldingoutput voltage.

The invention involves a novel topography for a power source wherein thepulse width modulated inverter is merely a second stage for creating anisolated fixed output DC bus without a feedback signal to the secondstage pulse width modulated inverter. This isolated bus is used in athird stage regulated by the actual welding parameters to create acurrent suitable for welding. Consequently, the invention involves anunregulated second stage not only providing necessary isolation but alsoto producing a fixed DC output bus to be used by a third stage whereinwelding regulation is accomplished. The unregulated second stageinverter is operated at a very high frequency with a duty cycle that isfixed during operation of the power source. The frequency is over 18 kHzand preferably about 100 kHz. The duty cycle is fixed at various levels;however, the preferred duty cycle is close to 100% to give the maximumefficiency level obtained by use of the present invention. The use of afixed, high duty cycle minimizes the current circulation time of thephase shift modulator controlled inverter second stage to substantiallyreduce heat an increase efficiency. In accordance with an aspect of theinvention, the output of the second unregulated inverter stage is arectifier using well known synchronous rectifier devices, which devicesare controlled by the secondary winding of the internal isolationtransformer of the second stage unregulated inverter. By usingsynchronous rectifier devices at the output of the second stage, thereis further improvement in the total efficiency of the power source. Byusing the present invention, the first stage is either an inputrectifier or an input rectifier with a power factor correctingconverter. A first stage power factor correcting converter is preferred.This converter is after a standard rectifier or can be combined with therectifier. Of course, this converter can be a passive power factorcorrecting converter or an active converter such as a boost, buck orbuck+boost converter. The first stage of the invention produces a firstDC bus with a fixed voltage. By using a standard first stage for thepower source, the first DC output signal which is the input DC bus tothe unregulated inverter can be regulated and fixed at a value of about400-900 volts DC. The output of the unregulated, isolation inverterforming the second stage of the novel power source is a fixed DC bushaving a fixed relationship with the input DC bus from the first stage.The voltage of the second DC bus or output is substantially less thanthe voltage of the DC bus from the first stage. The power source thusproduces a second DC bus which has a fixed mathematical relationshipwith the input DC bus from the power factor correcting converter. Inaccordance with standard practice, the second stage unregulated inverterincludes an isolation transformer having a primary winding and asecondary winding so that the secondary winding is isolated from theinput of the power source. The unregulated, second stage inverter can beoperated at a switching frequency to optimize the operation of thesecond stage inverter. Thus, extremely high switching frequency is usedto reduce the size and cost of the components in the novel, unregulatedsecond stage inverter. By utilizing a fixed duty cycle with phase shiftcontrol, voltage and current surges in the switching devices are reducedto provide a soft switching operation. Indeed, in the preferredembodiment, the duty cycle is fixed at 100% so that the switches arefull on or full off. This drastically reduces the circulated current inthe second stage and greatly improves the operating characteristics ofthe second stage inverter which also provides the function of isolatingthe welding output of the power source from the AC input of the powersource. By having the switching devices in the second stage unregulatedinverter operated at full on, this inverter has a high efficiency and isvery flexible in operation. An isolation transformer determines therelationship between the fixed DC bus at the input side of theunregulated second stage (a “first DC output signal” from the firststage) and the DC output bus at the output of this second stage (a“second DC output signal”). In some prior art power sources, the dutycycle at the primary winding of the isolation transformer in theregulated inverter is regulated by the welding operation. There is noregulation by the welding operation in either the first stage or secondstage of the novel power source used in the present invention.

The present invention involves a three stage power source where thesecond stage is unregulated isolation stage and the third stage is aregulated device, such as an inverter or chopper. A chopper with aswitching frequency less than the second stage inverter is preferred andis used in the practical implementation of the present invention. Thechopper better controls the output welding characteristics of the powersource. An inverter with an unregulated isolation stage followed by achopper stage provides a more efficient power source than a single stage(regulated) inverter, as used in prior art inverter based power sources.A power source where the second stage is an inverter has a maximumoutput that is at least twice the rated operating voltage of theinverter. When there are only two stages, a single stage regulatedinverter forms the second and last stage and runs at 50% or less energytransfer time. Therefore, the inverter of a prior art two stage powersource requires at least twice the primary current than the novel threestage power source of the invention. The unregulated isolated stage ofthe present invention has a higher efficiency even though it uses thesame transformer turn ratio. This is especially important when thesecond stage is running at full on or 100% duty cycle. Consequently, thepresent invention drastically increases the energy transfer time andreduces primary current of the isolation transformer in the second stageof an inverter based power source used for electric arc welding.

Normal control of an inverter in a power source used for welding is aphase-shift PWM control where the conduction states of the leading andlagging switches of the primary side have overlap that determines theenergy transfer time of the inverter. The primary current has tocirculate during the non-transfer time in order to achieve softswitching. Therefore, the primary side of the unregulated inverter usedin the invention is more efficient than that of the single stageregulated inverter used in the prior art, due to a small amount of offtime and less primary current in the unregulated inverter. This is anadvantage of using the present invention. Furthermore, the switchingfrequency of the inverter isolation stage, or second stage, ispreferably much higher than the switching frequency of the outputchopper third stage when using the present invention. The switching lossin the IGBT, and the diodes of the chopper used in the present inventionis less than the loss in the output rectifier diodes of the single stageinverter used as the second, regulating stage of a standard inverterbased power source. The unregulated second stage, as used in the presentinvention, is preferably a full bridge inverter; however, various otherinverter designs can be employed, especially dependent upon the voltageat the primary side DC bus. The preference is to use 600 volt IGBTswitches for either 450 volt bus or two 450 volt buses in series. Sincethe second stage inverter has no control feedback and is driven at fullon regardless of the output load demand, the secondary bus voltage isalways equal to the primary bus voltage divided by the transformer turnratio. This is another advantage of using the three stage inverter basedpower source for electric arc welding wherein the second stage is anunregulated inverter and the third stage is the regulated device,preferably a chopper but alternatively is an inverter.

The energy loss in the secondary of the unregulated inverter is abouttwice as much as the energy loss on the primary side when standarddiodes are used for the output rectifier of the second stage. The reasonfor this higher loss is that the secondary current is substantiallygreater than the primary current when the isolation transformer has aturn ratio with the primary turns substantially greater than thesecondary turns. Thus, loss in the output rectifier is much higher thanthe conduction loss in the primary switches of the second stageinverter. To reduce this situation, an aspect of the invention is theuse of very low on-resistance FETs configured as synchronous rectifierdevices. The use of synchronous rectifier devices in the output of thesecond stage reduces the conduction loss in the secondary side of theinverter. Diodes in the secondary side are hard switched even though theswitches in the primary side are soft switched. Soft switching is byphase-shift control of the switches in the primary side of the secondstage inverter. The reverse recovery current experiences more loss inthe secondary diodes than the on-resistance loss when the second stageinverter is switched at a frequency above 100 kHz. Therefore, it is alsodesirable to use synchronized rectifier devices to reduce switchinglosses by having a time delay between the primary switching control andthe secondary synchronous rectifier control. The ability to use thisconcept is an advantage of using synchronous rectifier devices in theoutput rectifier on the secondary side of the inverter forming thesecond stage of the three stage power source of the present invention.In addition, the use of synchronous rectifier devices in the secondaryside of the inverter reduces the energy loss experienced in the totaltransfer of power from the input DC bus to the output DC bus of thesecond stage. It has been established that the control of thesynchronous rectifier is simplified if the second stage unregulatedinverter is used to run full on at all times. This is the normaloperating condition of the unregulated inverter used as the second stagein the novel power source of the present invention. The secondaryvoltage in the unregulated inverter is used to generate the gate drivingsignals for the synchronous rectifier devices by connecting the devicesto opposite ends of the secondary winding of the isolation transformerused in the second stage unregulated inverter of the invention.Comparing synchronous rectifier devices to standard diodes, thesynchronous rectifier devices may with low-resistance FETs reduces theenergy lost in the secondary side of the unregulated inverter. Thisreduction is as great as 50%.

A power source for electric arc welding having an active power factorcorrecting feature and tight output control of the energy directed tothe welding operation requires at least two switching stages. These twostages assure that instantaneous energy transferred into the powersource and transferred out the power source can be regulatedindependently with appropriate energy storage components. Thus, a powerfactor correcting power source for electric arc welding generallyrequires two independent switching control circuits. One of the controlcircuits is used to control the energy or the output current for thewelding operation. The other control circuit is used to control the DCsignal from the active power factor correcting converter forming thefirst stage of the power source. Thus, electric arc welding powersources having power factor correcting capabilities requires twoswitching networks each of which has independent control requirements.The first switching control is for the output welding current and theother switching control is for power factor correcting at the inputstage of the power source. This second switching control assures thatthe output of the first stage is a fixed DC voltage referred to as a “DCbus.” The voltage of the DC bus itself is used to control the firststage converter to assure that the DC bus from this converter has afixed voltage level. To recapitulate an inverter based power source forelectric arc welding requires two separate switching networks and twocontrol circuits for these networks.

An inverter based power source for electric arc welding has anotherconceptual requirement. One of the stages in the power source mustprovide electrical isolation between the variable input AC signal andthe regulated output current suitable for welding. The isolation deviceis normally in the form of a transformer. In prior art, two stageinverter based power sources there are two locations for the isolationdevice. In the first example, the power factor correcting input stage isnot isolated and an isolation transformer is provided in the secondstage regulated output inverter. In another example, isolation is in thefirst stage power correcting converter. In this second example, anon-isolation output inverter or other non-isolation converter can beused as the second stage. The first example is more efficient than thesecond example due to 60 Hz effect on the RMS current at the input sideof the power source. In recapitulation, the second conceptualrequirement of a welding power source is isolation.

The two requirements of an active power factor correcting power sourcefor welding are (a) two separate and independent control circuits fortwo separate switching networks and (b) an appropriate structure forisolating the input of the power source from the output of the powersource. These basic requirements of inverter based power sources areimplemented in the present invention. When an unregulated inverter isused, the unregulated second stage is an isolation stage between tworegulated non-isolation stages to form a unique arrangement involving athree stage inverter based power source. The three stage inverter of thepresent invention is more efficient than the two stage inverter basedpower source assuming the same power factor correcting preregulator isused in both inverters. Thus, the present invention is more efficient,but still has the essential characteristics required for a power sourceused in electric arc welding. There are two independently controlledswitching networks. There is an isolation stage. These constraints areaccomplished in a manner to increase efficiency and obtain betterwelding performance and better heat distribution of the power switchingcomponents.

Since the second unregulated inverter stage of the present inventionprovides system isolation, many types of non-isolated converters can beused as the power factor correcting preregulator. A boost converter isthe most popular converter due to the current shaping function and thecontinuous line current characteristics of this type of conversion.However, the output voltage of the boost converter is higher than thepeak of the highest line voltage, which peak can be as high as 775volts. Thus, other active power factor correcting regulators can be usedwith the invention, which is a three stage power source wherein thesecond stage is unregulated and provides isolation. One of the otheroptions for the active power factor correcting input or first stage is astep-up/step-down converter so that the primary voltage bus or input busto the second stage can be lower than the peak of the input AC voltagesignal to the power source. This type of power factor correctingconverter still produces low harmonics. One such power factor converteris referred to as a buck+boost converter. A 400 volt to 500 volt DC busused for the second stage is obtained with an input AC voltage in therange of 115 volts to 575 volts. Irrespective of the AC voltage to thefirst stage, the output voltage of the active power factor converter iscontrolled to be at a level between 400 volts and 500 volts. Other typesof active and passive power factor correcting inverters can be used inthe invention. The preferred converter is active thus constituting asecond switching network requiring a second control circuit. When usingthe term electric arc welding, it also includes other output processes,such as plasma cutting.

As so far explained, the invention involves a three stage power sourcefor electric arc welding. A feedback control in the third stage createsan output current suitable for welding. The input first stage isnormally an active power factor correcting converter requiring a secondswitching network and a second independent control circuit. This threestage topography is not used in the prior art. By having thistopography, the added second stage is merely used to convert the highvoltage DC bus at the primary side of the second stage to a lowervoltage DC bus at the secondary side of the second stage isolated fromthe primary side. Thus, the invention involves a DC bus at the secondaryside of the second stage so that the bus can be used for regulation ofwelding power. The term “bus” means a DC signal that has a controlledfixed level. In the present invention, there is a first DC bus from theinput stage called the “first DC output” which first DC output has acontrolled DC voltage. There is a second DC bus at the secondary side ofthe second stage called the “second DC output” which second DC output isalso a controlled DC voltage level. The creation of a second DC bus atthe secondary side of an unregulated inverter has advantages, other thanthe advantages associated with the use of the unregulated second stageinverter as so far described. The secondary DC bus or second DC outputis isolated from the primary side of the second stage so that there isno isolation required in the third stage welding control circuit. Inother words, the output control circuit, such as a chopper, has an inputDC bus with a fixed voltage level. In practice, the chopper has acontroller with a control voltage that is derived from the input DC tothe chopper. This input DC signal is isolated from the input power.Consequently, the control voltage for the controller of the output stageor chopper can be derived from a non-isolated DC source. This isnormally the input signal to the chopper. Separate isolation of thecontrol voltage for the controller used in the output stage is notrequired. The use of a fixed DC bus from the second stage allows the DCvoltage to the output third stage, which is regulated by the weldingoperation, to be much lower than the normal input primary DC bus (“firstDC output”) of the power source. In the past, the output of the powerfactor converter is a relatively high level DC signal based upon the useof a boost converter. This high DC voltage was directed to the regulatedinverter stage for use in outputting a current suitable for the welding.By using the present invention the high voltage from the output bus ofthe power factor converter is drastically reduced. It is more efficientto convert a 100 volt DC bus into a 15 volt control power than toconvert a 400 volt DC bus to a 15 volt control power. This creation of asecond, lower voltage DC bus is a substantial advantage of the threestage power source of the present invention.

In accordance with the present invention there is provided a powersource for an electric arc welding process wherein the power sourcecomprises an input stage having an AC input and a first DC outputsignal. A second stage in the form of an unregulated DC to DC converterhas an input connected to the first DC output signal and an output inthe form of a second DC output signal electrically isolated from thefirst DC output signal with a magnitude of a given ratio to the first DCoutput signal. The power source includes a third stage to convert thesecond DC output signal to a welding current for the welding process. Inaccordance with another aspect of the present invention there isprovided a power factor correcting converter as the first stage of thenovel three stage power source. The third stage of the power sourceincludes a regulated converter such as a chopper or inverter. When usingan inverter, the output is a DC signal directed to a polarity network orswitch, which switch allows DC welding by the power source. The polarityswitch allows welding either DC negative, DC positive or AC. The weldingprocess, using either a chopper or an inverter, can be performed withshielding gas, such as MIG welding, and can use any type of electrode,such as tungsten, cored wire or solid metal wire. In accordance with anaspect of the invention, the output of the unregulated DC to DCconverter is substantially less than the input to the second stage. Inmost instances, the input and output of the second stage are DC voltageswith generally fixed magnitudes.

The present invention relates to a three stage power source for electricarc welding wherein the first stage is normally regulated to produce afixed first DC signal or “first DC output.” This first fixed DC signal,DC bus or DC output is connected to the primary side of a second stageunregulated inverter, wherein the secondary side of the inverter is afixed DC output signal directed to an output third stage. The thirdstage has a regulated switching network for creating a current suitablefor welding. The regulated third stage is controlled by a feedback loopfrom the welding operation to control the welding operation by current,voltage or power.

In accordance with another aspect of the invention, the switchingfrequency of the unregulated inverter constituting the second stage ofthe power source is substantially greater than 18 kHz. Indeed, theswitching frequency of the second stage is substantially greater thanthe switching frequency of the regulated third stage of the power sourceand is normally about 100 kHz. In this manner, smaller components areused in the second stage of the three stage power source.

The primary object of the present invention is the provision of a threestage power source for electric arc welding, which power source isefficient in operation, has a better welding performance and is morerobust.

Still a further object of the present invention is the provision of anelectric arc power source as defined above, which electric arc powersource has a second stage where a DC bus is converted to a DC busthrough an isolation transformer without regulation of the conversion.

Another object of the present invention is the provision of a method forconverting an AC signal into a DC current suitable for welding whichmethod involves rectifying and converting said AC input into a DC signalcalled a first DC output. The DC signal is converted to an isolated DCbus and then used to power a regulated stage controlled by theparameters of a welding operation to produce a current suitable forwelding.

Another object of the present invention is the provision of a powersource and method, as defined above, which power source and method usesa high switching frequency above 18 kHz and about 100 kHZ with a fixedduty cycle to prevent losses.

Yet another object of the present invention is the provision of a powersource for electric arc welding and method, as defined above, whichpower source and method have reduced magnetic losses, reduced componentsizes, and provides an efficient power source for electric arc welding.

Still a further object of the present invention is the provision of apower source for electric arc welding and method of using same, whichpower source and method involves an unregulated DC to DC converterproviding isolation so the output stage regulated for welding need notinclude an isolation feature.

Another object of the present invention is the provision of a powersource for electric arc welding and a method of using the same, whichpower source and method includes a DC to DC unregulated inverter wherethe output of the inverter is much lower in voltage than the input tothe inverter. Thus, the efficiency is increased due to outputting alower voltage that needs to be converted to a welding voltage.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a three stage power source anddisclosing an embodiment of the invention;

FIG. 2 and FIG. 3 are block diagrams similar to FIG. 1 disclosingfurther embodiments of the invention;

FIGS. 4-8 are partial block diagrams illustrating a power sourceconstructed in accordance with the invention and disclosing differentfirst stage embodiments;

FIG. 9 is a block diagram showing the last two stages of a power sourceconstructed in accordance with the present invention wherein the outputstage provides AC welding current;

FIG. 9A is a block diagram of a waveform technology control circuit foruse in the embodiment of the invention illustrated in FIG. 9 and usablein various implementations of the present invention together with graphsshowing three welding waveforms;

FIG. 10 is a block diagram illustrating a second and third stage of apower source constructed in accordance with the present inventionwherein the output stage is DC welding current;

FIG. 11 is a block diagram illustrating the topography of a three stagepower source constructed in accordance with the present invention forcreating current suitable for electric arc welding with two separatecontroller control voltage supplies;

FIG. 12 is a block diagram illustrating a specific power sourceemploying the topography of the present invention;

FIGS. 13-16 are wiring diagrams illustrating four different circuits forcorrecting the power factor in the first stage of a power sourceconstructed in accordance with the present invention;

FIG. 17 is a combined block diagram and wiring diagram illustrating thepreferred embodiment of the unregulated inverter constituting the novelsecond stage of a power source constructed in accordance with thepresent invention; and,

FIGS. 18-21 are wiring diagrams showing several inverters used as thesecond stage unregulated, isolation inverter comprising the novel aspectof the present invention.

PREFERRED EMBODIMENT

The present invention relates to a three stage power source for use inelectric arc welding. Of course, the concept of welding also encompassesthe related technology of plasma cutting. The invention has an inputstage for converting an AC signal into a first DC output bus. Thisoutput bus has a fixed voltage level in accordance with the preferredembodiment of the invention and is directed to the input of a novelsecond stage used in the welding technology and best shown in FIG. 16.This novel second stage is an unregulated inverter which includes anisolation feature and has a second DC output or second DC bus which isproportional to the DC input bus. The level relationship is fixed by theconstruction of the unregulated inverter. The unregulated inverter has aswitching network wherein the switches are operated at a high switchingfrequency greater than 18 kHz and preferably about 100 kHz. Theswitching frequency of the switch network in the unregulated inverterforming the second stage of the power source allows use of smallmagnetic components. The isolated DC output of the unregulated inverteris directed to a third stage of the power source. This third stage iseither a chopper or inverter which is regulated by a welding parameter,such as current, voltage or power of the welding operation.Consequently, the topography of the present invention has an input stageto produce a first DC signal, a second unregulated DC to DC stage toprovide an isolated fixed DC voltage or DC bus that is used by the thirdstage of the power source for regulating the current used in the weldingoperation. Three examples of the present invention are illustrated inFIGS. 1-3. Power source PS1 in FIG. 1 includes first stage I, secondstage II, and third stage III. In this embodiment, stage I includes anAC to DC converter 10 for converting AC input signal 12 into a first DCbus 14. The input 12 is an one phase or three phase AC line supply withvoltage that can vary between 400-700 volts. Converter 10 is illustratedas an unregulated device which can be in the form of a rectifier andfilter network to produce DC bus 14 identified as (DC#1). Since the ACinput signal is a line voltage, DC bus 14 is generally uniform inmagnitude. Unregulated inverter A is a DC to DC converter with anisolation transformer to convert the DC bus 14 (DC#1) into a second DCbus or second DC output 20 (DC#2). Output 20 forms the power input tostage III which is converter 30. The DC voltage on line 20 into acurrent suitable for welding at line B. A feedback control or regulationloop C senses a parameter in the welding operation and regulates thecurrent, voltage or power on line B by regulation of converter 30. Inpractice, converter 30 is a chopper, although use of an inverter is analternative. By having a three stage power source PS1 as shown in FIG.1, the switching network of the second stage has a frequency that isnormally higher than the switching frequency of converter 30.Furthermore, the DC voltage in line 20 (DC#2) is substantially less thanthe DC voltage from stage I on line 14 (DC#1). In practice, there is anisolation transformer in inverter A. The transformer has an input orprimary section or side with substantially more turns than the secondarysection or side used to create the voltage on line 20. This turn ratioin practice is 4:1 so that the voltage on line 20 is ¼ the voltage online 14.

The general topography of the present invention is illustrated in FIG.1; however, FIG. 2 illustrates the preferred implementation whereinpower source PS2 has essentially the same stage II and stage III aspower source PS1; however, input stage I is an AC to DC converter 40including a rectifier followed by a regulated DC to DC converter. Theconverted signal is a DC signal in line 14 shown as a first DC bus(DC#1). The voltage on line 14 is regulated as indicated by feedbackline 42 in accordance with standard technology. Thus, in power sourcePS2 the output welding converter 30 is regulated by feedback loop C. Thevoltage on line 14 is regulated by feedback loop shown as line 42. Sinceconverter 40 is a power factor correcting converter it senses thevoltage waveform as represented by line 44. By using power source PS2,the first DC bus 14 is a fixed DC voltage with different one phase orthree phase voltages at input 12. Thus, output 20 is merely a conversionof the DC voltage on line 14. DC#2 is a fixed voltage with a leveldetermined by the isolation transformer and the fixed duty cycle of theswitching network in unregulated inverter A. This is the preferredimplementation of the novel power source employing three separate anddistinct stages with stage II being an unregulated inverter forconverting a fixed first DC output or DC bus to a second fixed DC outputor DC bus used to drive a regulated welding converter, such as a chopperor inverter. As another alternative, stage I could be regulated by afeedback from the DC #2 bus in line 20. This is represented by thedashed line 46 in FIG. 2.

Power source PS3 in FIG. 3 is another implementation of the three stagepower source contemplated by the present invention. This is not thepreferred implementation; however, the three stage power source of thepresent invention can have the input converter 50 regulated by feedbackloop 52 from the welding current output B. With this use of a threestage power source, converter 50 is regulated by the welding output andnot by the voltage on line 14 as in power source PS2. With regulationfrom welding output B, converter 50 is both a power factor correctingstage and a welding regulator. However, this implementation of theinvention is disclosed for a complete technical disclosure of the threestage power source as contemplated by the present invention.

As previously described, input stage I converts either a single phase ora three phase AC signal 12 into a fixed DC bus 14 (DC#1) for use by theunregulated inverter A constituting second stage II. The implementationof the present invention generally employs a DC to DC converter in stageI to produce the DC voltage indicated as line 14 in FIGS. 1-3. The DC toDC converter of stage I can be selected to create the desired voltage online 12. Three of these converters are shown in FIGS. 4-6 wherein aninput rectifier 60 provides a DC voltage in lines 60 a, 60 b to a DC toDC converter which may be a boost converter 62, a buck converter 64 or abuck+boost converter 66, as shown in FIG. 4, FIG. 5 and FIG. 6,respectively. By using these converters, the DC to DC converter of stageI incorporates a power factor correcting chip, which chip allows thepower factor to be corrected thereby reducing the harmonic distortion atthe input of the power source. The use of a power factor correctinginput DC to DC converter is well known in the welding art and is used inmany prior art two stage topographies. The present invention is asubstantial improvement over such two stage power sources. Converters62, 64 and 66 preferably include a power factor correcting chip;however, this is not required for practicing the invention. The mainpurpose of stage I is to provide a DC bus (DC#1) in line 12, which busis indicated to be lines 14 a, 14 b in FIGS. 4-6 to produce a fixed DCbus (DC#2) in line 12 indicated by lines 20 a, 20 b in the same figures.Power factor correction is not required to take advantage of the threestage topography of the present invention. A non power factor correctinginput stage is illustrated in FIG. 7 where the output lines 60 a, 60 bof rectifier 60 are coupled by a large storage capacitor 68 to produce agenerally fixed voltage in lines 14 a, 14 b. Stage I in FIG. 7 does notincorporate a power factor correcting circuit or chip as preferred inimplementation of the present invention. However, the power source stillinvolves three stages wherein the second stage is unregulated isolatedinverter A to produce a generally fixed voltage on lines 20 a, 20 b.Another modification of input stage I is illustrated in FIG. 8 where apassive power factor correcting circuit 70 is connected to a three phaseAC input L1, L2 and L3 to produce a generally fixed DC voltage acrosslines 14 a, 14 b, which lines constitutes the DC bus 14 (DC#1) at theinput of inverter A. The disclosures of modified stage I in FIGS. 4-8are only representative in nature and other input stages could be usedin practicing the invention with either single phase or three phaseinput signal and with or without power factor correcting.

By providing low fixed voltage on output bus 20 illustrated as lines 20a, 20 b, the third stage of the novel three stage power source forwelding can be a chopper or other converter operated at a frequencygreater than 18 kHz. The switching frequencies of the unregulatedinverter and the regulated output converter may be different. Indeed,normally the switching frequency of the chopper is substantially lessthan the frequency of unregulated inverter A. Power source PS4 shown inFIG. 9 illustrates the use of the present invention wherein stage III isa standard regulated converter 100 of the type used for electric arcwelding. This converter is driven by fixed input DC bus 20 and isregulated by feedback from the welding operation 120 to provide currentsuitable for welding across output leads 102, 104. Leads 102 is apositive polarity lead and leads 104 is a negative polarity lead. Inaccordance with standard output technology for a two stage inverterbased power sources, leads 102, 104 are directed to a standard polarityswitch 110. This switch has a first position wherein lead 102 isdirected to the electrode of the welding operation 120 so the output ofpolarity switch 110 has a positive polarity on output line 10 a and anegative polarity on output line 10 b. This produces an electrodepositive DC welding process at weld operation 120. Reversal of polarityswitch network 110 can produce an electrode negative DC welding processat weld operation 120. Thus, a DC welding process with either DCnegative or DC positive can be performed according to the setting of thestandard polarity switch 110. In a like manner, polarity switch 110 canbe alternated between electrode negative and electrode positive toproduce an AC welding process at weld operation 120. This is standardtechnology wherein polarity switch 110 drives the DC output fromregulated converter 100 to produce either an AC welding process or a DCwelding process. This process is regulated and controlled by a feedbacksystem indicated as line or loop 122 directed to controller 130 forregulating converter 100 and for setting the polarity of switch 110 asindicated by lines 132, 134, respectively. By regulating the weldingoperation at stage III, the unregulated inverter at stage II can have arelatively higher switching frequency to reduce the component sizeswithin the second stage of the power source. The preferred embodiment ofthe present invention employs waveform control technology pioneered byThe Lincoln Electric Company of Cleveland, Ohio. This type of controlsystem is well known and is schematically illustrated in FIG. 9A whereincontrol circuit 150 processes a waveform profile as a voltage on line152 a is outputted from waveform generator 152. The waveform profile iscontrolled by feedback loop 122 as schematically illustrated by erroramplifier 154 having an output 156. Thus, the profile of the waveformfrom generator 152 is controlled by the feedback loop 122 and produces asignal in output line 156. This line is directed to an appropriate pulsewidth modulator circuit 160 operated at a high frequency determined bythe output of oscillator 162. This frequency is greater than 18 kHz andis often higher than 40 kHz. The regulated converter 100 preferablyoperates under 40 kHz. The output of the pulse width modulator, which isnormally a digital circuit within controller 130, is shown as line 132for controlling the waveform by way of regulated converter 100. Inaccordance with standard practice, the waveform of inverter 100 can haveany profile, either AC or DC. This feature is schematically illustratedas waveform 152 b, 152 c and 152 d at the right portion of FIG. 9A.Waveform 152 b is an AC waveform of the type used in AC MIG weldingwhere a higher negative electrode amperage is provided. A higherpositive amperage is also common. In waveform 152 c, the amperage forboth electrode negative and electrode positive is essentially the samewith the length of the negative electrode portion being greater. Ofcourse, a process for AC welding can be adjusted to provide balanced ACwaveforms or unbalanced AC waveforms, either in favor of electrodenegative or electrode positive. When polarity switch 110 is set foreither a DC negative or a DC positive welding operation, a pulse weldingwaveform, shown as waveform 152 d, is controlled by waveform generator152. Various other waveforms, both AC and DC, can be controlled bycontroller 130 so the welding operation 120 can be adjusted to be AC, orDC. Furthermore, the welding operation can be TIG, MIG, submerged arc orotherwise. Any process can be performed by power source PS4 or otherpower sources using the present invention. The electrode can benon-consumable or consumable, such as metal cored, flux cored or solidwire. A shielding gas may or may not be used according to the electrodebeing employed. All of these modifications in the welding operation canbe performed by using the present invention. A modification of powersource PS4 to perform only DC welding is illustrated as power source PS5in FIG. 10. In this power source, welding operation 120 performs only aDC welding operation so that feedback loop 122 is directed to controller170 having an output 172. Regulated converter 100 a is preferably achopper to produce a DC voltage across lines 102 a, 104 a. Controller170 is controlled by waveform generator 152, as shown in FIG. 9A. Thepolarity on lines 102 a, 104 a is either electrode negative or electrodepositive according to the demand of the DC welding process performed atwelding operation 120. Regulated converter 100 a is more simplified thanthe welding output of power supply PS4 shown in FIG. 9. FIGS. 9 and 10,together with the control network or circuit 150 shown in FIG. 9A,illustrates the versatility of the novel three stage power sourceconstituting the present invention.

In implementing either a two stage power source as used in the prior artor the novel three stage power source of the present invention, it isnecessary to provide a voltage for operating the controllers for boththe regulated and unregulated switching networks used in these two typesof power sources. FIG. 11 illustrates the architecture and schemeemployed in the preferred embodiment of the present invention to obtaincontrol voltages to operate the various controllers of a three stagepower source, such as power source PS6. The use of an output of apreregulator to provide the control voltage for the switching controllerof the preregulator and switching controller of the second stage of atwo stage power source is well known and is disclosed in Moriguchi U.S.Pat. No. 5,926,381, incorporated by reference herein. An output chopperfor performing a welding operation routinely obtains the controllercontrol voltage from the input DC voltage to the chopper. These two wellknown technologies are incorporated in power source PS6. The three stagepower source can be operated with controllers having power suppliesderived from various locations in the power source. Being more specific,power source PS6 has a first power supply 180 with an output 182 andinputs 184, 186 from the first DC bus on leads 14 a, 14 b (DC#1). Powersupply 180 includes a buck converter or flyback converter, not shown, toreduce the high voltage at the output of preregulator 40 of FIG. 2 to alow voltage on line 182. This control voltage on line 182 may be between5 and 20 volts. Voltage on line 182 is directed to a first controller190 having an output lead 192 for performing the operation ofpreregulator 40 in accordance with standard technology. The preregulator40 has regulation feedback lines 42, 44 shown in FIGS. 2 and 3, butomitted in FIG. 11. Unregulated inverter A does not require a controllerto modulate the duty cycle or the fixed relationship between the inputand output voltages. However, the unregulated inverter A receives acontrol signal on an output lead 198 from a second controller 194 thatreceives controller operating voltage in line 196 from the first powersupply 180. This arrangement is similar to the concept disclosed inMoriguchi U.S. Pat. No. 5,926,381, except second stage controller 194 isnot a regulating controller as used in the two stage power source of theprior art. As an alternative, a third power supply PS#3 is driven by onephase of input 12 to give an optional power supply voltage to a firstcontroller 190 shown as dashed line 176. Regulated output converter 30of stage III has a second power supply 200 labeled PS#2 coupled to thesecond DC bus leads 20 a and 20 b via inputs 206 and 204, respectively,with a controller voltage on line 202 determined by the voltage on DCbus 20 (DC#2) illustrated as including leads 20 a, 20 b. Again, powersupply 200 includes a buck converter or flyback converter to convert theDC bus at the output of unregulated converter A to a lower voltage foruse by controller 210 having an output 212. The signal on line 212regulates the output of welding converter 30 in accordance with thefeedback signal on line C, as discussed with respect to power sourcesPS1, PS2 in FIGS. 1 and 2, respectively. DC bus 14 (DC#1) and DC bus 20(DC#2) provides input to power supplies 180, 200 which are DC to DCconverters to produce low level DC control voltage for controllers 190,194 and 210. As an alternative shown by dashed line 220, the first powersupply 180 labeled PS#2 can provide a control voltage for the thirdcontroller 210. FIG. 11 has been disclosed to illustrate the versatilityof using a three stage power source with controllers that can receivereduced supply voltages from various fixed DC voltage levels indicatedto be PS#1 and PS#2. Other arrangements could be employed for providingthe controller voltage, such as a rectified connection to one phase ofAC input voltage l2 via lines 272 and 274 by a transformer in a mannerillustrated as PS#3.

Another implementation of the present invention with more specificdetails on the preferred embodiment of the present invention isillustrated in FIG. 12 wherein power source PS7 is similar to powersource PS6 with components having the same identification numbers. Inthe preferred embodiment of the present invention, the output stage IIIis a chopper 230 for directing a DC current between electrode E andworkpiece W. Current shunt S provides the feedback signal C tocontroller 210. High switching speed inverter 240 of stage II hascharacteristics so far described with the isolation provided bytransformer 250 having primary winding 252 and secondary winding 254.The primary side of DC to DC converter 240 is the switching networkdirecting an alternating current to primary winding 252. The rectifiedoutput from secondary 254 is the secondary section or side of converter240. Converter 240 employs a high switching speed inverter that has aduty cycle or phase shift set by controller 194. The switching frequencyis about 100 kHz in the practical version of this power source. The dutycycle remains the same during the welding operation by chopper 230;however, in accordance with the invention, the duty cycle or phase shiftof the inverter may be adjusted as indicated by “ADJ” circuit 260 havingan output 262 for adjusting controller 194. In the preferred embodiment,the duty cycle is close to 100% so that the switch pairs are conductivetogether their maximum times at the primary side of inverter 240.However, to change the fixed relationship between the first DC bus 14and the second DC bus 20, circuit 260 can be used to adjust the dutycycle or phase shift. Thus, the unregulated, isolation inverter 240 ischanged to have a different, but fixed duty cycle. However, the dutycycle normally is quite close to 100% so the switch pairs are operatedessentially in unison. The duty cycle probably varies between 80-100% innormal applications of the present invention. In the preferredimplementation, boost converter 62 shown in FIG. 4 is used for a powerfactor correcting input stage I. This boost converter is operated inaccordance with controller 190 having a control voltage 182 aspreviously described. In accordance with a slight modification of thepreferred embodiment, supply 270 has a transformer connected by lines272 and 274 across one phase of a single phase or three phase AC input12. A rectifier and filter in power supply 270 produces a low controlvoltage in optional dashed line 276 for use instead of the controlvoltage in line 182 if desired. These two alternatives do not affect theoperating characteristics of power source PS7. Other such modificationsof a three stage power source for electric arc welding can be obtainedfrom the previous description and well known technology in the weldingfield.

Input stage I normally includes a rectifier and a power factorcorrecting DC to DC converter as disclosed in FIGS. 4-8. These inputstages can be used for both three phase and single phase AC signals ofvarious magnitudes, represented as input 12. Certain aspects of an inputstage for three phase AC input power are disclosed with respect to thecircuits in FIGS. 13-16. Each of these circuits has a three phase inputand a DC bus output (DC#1) that is obtained with a low harmonicdistortion factor and a high power factor for the input stage. Thedisclosure in FIGS. 1-12 are generally applicable to the novel threestage power source; however, the particular stage I used is relevant toboth a two stage power source of the prior art or a three stage powersource of the present invention. In FIG. 13, the input circuit 300 ofstage I includes a three phase rectifier 302 with output leads 302 a,302 b. Boost switch 310 is in series with an inductor 312, diode 314 anda parallel capacitor 316. An appropriate circuit 320 which is a standardpower factor correcting chip has an input 322 to determine the inputvoltage, a regulation feedback line 322 a and an output 324 foroperating the boost switch to cause the current in input 12 to begenerally in phase with the input voltage. This chip is a standard threephase power factor correcting boost converter chip that can be used inthe present invention and is also used for a normal two stage powersource. In a like manner, input circuit 330 shown in FIG. 14 has a threephase rectifier 302 with output leads 302 a, 302 b as previouslydescribed. A boost circuit including inductor 350, diodes 352, 354 andcapacitors 356, 358 are used in conjunction with switches 340, 342 toprovide coordination of the current at the output of circuit 330 andinput voltage 12. To accomplish this objective, a standard chip 360provides gating pulses in lines 362, 364 in accordance with the sensedvoltage in input 366 and feedback regulation signals in lines 367, 368.This is standard technology to provide three phase power factorcorrection of the type that forms the input of a two stage power sourceor the novel three stage power source of the present invention. It hasbeen found that the active three phase circuits 300, 330 when operatedon a three phase input provide an input power factor of about 0.95. Thepower factor of a stage I when having a single phase AC input can becorrected upwardly to about 0.99. Since a three phase power source cangenerally be corrected only to a lower level, it has been found that apassive circuit for the input stage I of a two stage or three stagepower source is somewhat commensurate with the ability of an activepower factor correcting circuit. A standard passive circuit 400 is shownin FIG. 15, wherein each of the three phases is rectified by three phaserectifier 302 which directs DC current through output leads 302 a, 302 bto a filter circuit including inductor 412 and capacitor 414. It hasbeen found that a passive circuit such as shown in FIG. 15 can correctthe power factor of the three phase input to a level generally in therange of about 0.95. This is somewhat the same as the ability of anactive circuit for a three phase input circuit. A buck+boost inputcircuit 420 is shown in FIG. 16. Rectified current on lines 302 a, 302 bis first bucked by switch 422 using standard power factor correctingchip 430 having a line 432 having a voltage waveform signal from input12, that also steers chip 434 to operate boost switch 440. Switches 422,440 are operated in unison to control the input power factor using acircuit containing inductor 450, diode 452 and capacitor 454. Circuits300, 330, 400 and 420 are standard three phase passive power factorcorrecting circuits using standard technology and available switchescontrolled by the input voltage waveform and the current of DC#1. FIGS.13-16 are illustrative of certain modifications that can be made to thefirst stage of the three stage power source of the present invention. Ofcourse, there is other technology for improving the power factor andreducing the harmonic distortion of both DC and AC signals of the typeused to drive power sources of electric arc welders. Any standardcircuit can be incorporated in the present invention in the same manneras they are used in other power sources that do not employ the novelconcepts of the present invention.

Unregulated inverter A of stage II can use various inverter circuits.The preferred circuit is illustrated in FIG. 17 wherein the inverter isdivided between a primary section or side defined by the input toprimary winding 252 of isolating transformer 250 and a secondary sectionor side defined by output of secondary winding 254. Referring first tothe primary section or side of inverter A, full bridge circuit 500 isemployed wherein paired switches SW1-SW3 and SW2-SW4 are acrosscapacitor 548 are connected by leads 502, 504, 506, and 508. Theswitches are energized in alternate sequence by gating pulses on lines510, 512, 514, and 516, respectively. Controller 194 outputs gatingpulses in lines 510-516 and an adjusted duty cycle determined by thelogic on line 262 from circuit 260 as previously discussed. The dutycycle is controlled by changing the phase shift of lines 510 and 512 adlines 514 and 516. Circuit 260 adjusts the duty cycle or phase shift ofthe paired switches. This adjustment is fixed during the operation ofinverter A. In practice, circuit 500 has about 100% duty cycle or phaseshift, where each pair of switches has maximum periods of overlappingconduction. Controller 194 has a control voltage from an appropriatesupply indicated by line 196, as also previously described. In operationof circuit 500, an alternating current is directed through primarywinding 252. This current has an ultra high frequency normally at leastabout 100 kHz so the components can be reduced in size, weight and cost.The high switching frequency is not dictated by the welding operation,but is selected for efficiency of unregulated stage A of the three stagepower source. The secondary section or side of inverter A is a rectifier520 having synchronous rectifier devices 522, 524 with outputs 542 and540, respectively. Synchronous rectifier devices are well known in thegeneral electrical engineering art and are discussed in Boylan U.S. Pat.No. 6,618,274 incorporated by reference herein. These devices are gatedby signals on lines 526, 528 created at the opposite ends of secondarywinding 254 in accordance with standard technology. Leads 530, 532, 534,540, and 542 form the output leads of rectifier 520 to create a DCvoltage (DC#2) across leads 20 a, 20 b. The current is smooth by a choke544 and is across capacitor 546, in accordance with standard weldingtechnology. Inverter A is unregulated which means that it is notadjusted by a real time feedback signal from the welding operation. Itmerely converts DC bus 12 (DC#1) to DC bus 20 (DC#2). This conversionallows a substantial reduction in the voltage directed to the regulatedthird stage of the power source using inverter A. The reduction involtage is primarily determined by the turns ratio of transformer 250,which ratio, in the preferred embodiment, is about 4:1. Thus, the fixedvoltage on output bus 20 is about ¼ the fixed voltage on output bus 12of the first stage. Several advantages of an unregulated stage arecontained in an article entitled The incredible Shrinking (Unregulated)Power Supply by Dr. Ray Ridley incorporated by reference herein asbackground information. A basic advantage is the ability to increase thefrequency to above 100 kHz to reduce the size and cost of the inverterstage.

Various circuits can be used for the unregulated inverter A constitutingnovel stage II of the invention. The particular type of inverter is nota limiting feature of the invention. Several inverters have been used inthe invention. Some are illustrated in FIGS. 18-21. In FIG. 18, inverterA is shown as using a full bridge circuit 600 on the primary side oftransformer 250. A switch and diode parallel circuit 602, 604, 606 and608 are operated in accordance with the standard phase shift full bridgetechnology, as explained with respect to the inverter A version shown inFIG. 17. A modification of the internal workings for inverter A isillustrated in FIG. 19 utilizing a cascaded bridge with series mountedswitch circuits 610, 612 and 614, 616. These switch circuits areoperated similar to a half bridge and include input capacitors 548 a,548 b providing energy for the switching circuits which in parallel iscapacitor 620 and is in series with diode 622, 624. The two switchcircuits are in series so there is a reduced voltage across individualswitches when a phase shift control technique similar to the techniquefor the full bridge inverter of FIG. 17 is used. This type of inverterswitching network is illustrated in Canales-Abarca U.S. Pat. No.6,349,044 incorporated by reference herein showing an inverter using acascaded bridge, sometimes referred to as a three level inverter. Adouble forward inverter is shown in FIG. 20 wherein switches 630, 632provide a pulse in section 252 a of the primary winding for transformer250 a. In a like manner, switches 634, 636 are operated in unison toprovide an opposite polarity pulse in primary section 252 b. Thealternating pulse produces an AC at the primary winding of transformer250 a to produce an isolated DC output in secondary winding 254. Astandard half bridge circuit is shown as the architecture of inverter Ain FIG. 21. This half bridge includes switches 640, 642 alternatelyswitched to produce an AC in primary winding 252 of transformer 250.These and other switching circuits can be used to provide an AC signalin the primary winding of transformer 250 so that the secondary isolatedAC signal is rectified and outputted on leads 20 a, 20 b as DC#2. Themere description of certain representative standard switching networksis not considered to be exhaustive, but just illustrative.

The invention involves a power source for electric arc welding whereinthe control of the welding current is not performed in the second stage.In this stage, a DC bus having a high voltage is converted to a fixed DCbus (DC#2) having a low voltage for the purposes of driving a thirdstage, which third stage is a regulated stage to provide a currentsuitable for electric arc welding. Electric arc welding incorporates andis intended to include other welding related applications such as theconcept of plasma cutting. The various circuits used in the three stagescan be combined to construct various architectures for the basictopography which is a three stage power source.

1. A power source for an electric arc welding process having a topologywith each stage of the topology defined by an input and an outputsignal, said power source comprising: an input stage being a converterhaving an AC input signal and a first fixed DC output signal, having afirst magnitude, said first fixed DC output signal being on a first DCbus; a second stage in the form of an unregulated DC to DC converterhaving as an input signal said first fixed DC signal on said first bus,the second stage being unregulated such that the second stage is notadjusted by a real time feedback signal from the welding process andincluding: a network of switches switched at a high frequency with agiven duty cycle to convert said first fixed DC input signal of saidsecond stage into a first internal high frequency AC signal, said dutycycle being fixed during operation of said power source; an isolationtransformer with a primary winding and a secondary winding for creatinga second internal AC signal different than the first AC signal; and arectifier to convert said second internal AC signal into a second fixedDC output signal not used for welding having a second magnitude relatedto said duty cycle of said switches, said second magnitude being lessthan said first magnitude, said second fixed DC output signal being on asecond DC bus; and a third stage regulated converter which receives saidsecond fixed DC signal as an input signal and is regulated by a feedbacksignal from the welding process, said third stage to convert said secondfixed DC signal to an output signal used for welding, said third stageconverting said second fixed DC signal separate and distinct of saidinput and second stages.
 2. A power source as defined in claim 1 whereinsaid input stage includes a rectifier and a power factor correctingconverter.
 3. A power source as defined in claim 2 wherein said powerfactor correcting converter is a buck+boost converter.
 4. A power sourceas defined in claim 2 wherein said power factor correcting converter isa two level converter.
 5. A power source as defined in claim 2 whereinsaid power factor correcting converter is an active converter.
 6. Apower source as defined in claim 1 wherein said AC input is a threephase or a single phase input.
 7. A power source as defined in claim 1wherein said input stage comprises a regulated converter stage having acontroller with a control voltage and a voltage circuit to provide saidcontrol voltage from said first DC output signal.
 8. A power source asdefined in claim 7 wherein said input regulated converter stage includesa feedback circuit from said first fixed DC signal to regulate saidinput converter stage.
 9. A power source as defined in claim 7 whereinsaid input regulated converter stage includes a feedback circuit fromsaid second fixed DC signal to regulate said input converter stage. 10.A power source as defined in claim 1 wherein said network of switches isa full bridge inverter.
 11. A power source as defined in claim 1 whereinsaid duty cycle is fixed at about 100%.
 12. A power source as defined inclaim 1 wherein said duty cycle is adjustable when the power source isnot operating.
 13. A power source as defined in claim 1 wherein saidhigh switching frequency is greater than about 18 kHz.
 14. A powersource as defined in claim 1 wherein said primary winding hassubstantially more turns than said secondary winding.
 15. A power sourceas defined in claim 1 wherein said third stage is a chopper.
 16. A powersource as defined in claim 1 wherein said power source has an outputpower capacity of at least 5 KW.
 17. A power source as defined in claim1 including a circuit to regulate said third stage by said currentsuitable for welding.
 18. The power source of claim 1, wherein saidfirst fixed DC output signal is regulated and controlled to a voltagebetween 400 and 500 volts.
 19. The power source of claim 18, whereinsaid second fixed DC output signal is about ¼ the voltage of said firstfixed DC bus.
 20. A method of providing a regulated welding output froma first AC power supply input signal to a power source topology witheach stage of the topology defined by an input and an output signal,said method comprising: (a) converting said first AC supply input signalto a first stage of the topology into a first fixed DC bus voltageoutput signal on a first DC bus; (b) converting said first fixed DC busvoltage as an input signal to a second stage of the topology into asecond AC signal using a network of switches of said second stage, saidnetwork of switches having a fixed duty cycle defined by a controlleroutput that is unregulated so as not to be adjusted by a real timefeedback signal from the welding output; (c) rectifying said second ACsignal to a second fixed DC bus voltage output signal not used forwelding on a second DC bus where said second fixed DC bus voltage is theproduct of the first DC bus multiplied by a constant such that thesecond fixed DC bus voltage is less than the first fixed DC bus voltageand said second DC bus is isolated by said second stage from said firstDC bus; (d) converting said second fixed DC bus voltage as an inputsignal to a third stage of the topology into an output signal of thethird stage used for welding, wherein said second fixed DC bus voltageis converted separate and distinct of said first and said second stages;and (e) regulating said output signal of said third stage with afeedback signal from said welding output such that said output signal ofsaid third stage is a desired regulated welding output.
 21. A method asdefined in claim 20 wherein said first stage further includes: (f) powerfactor correcting of said first stage.
 22. A method as defined in claim20 wherein said second stage is a high switch frequency inverter andincludes an isolation transformer.
 23. A method as defined in claim 20wherein said second stage includes an isolation transformer with a turnratio and said constant is at least partially determined by said turnratio.
 24. A method as defined in claim 20 wherein said second stageincludes a switching network wherein said switches of said network havea fixed duty cycle.
 25. A method as defined in claim 24 wherein saidfixed duty cycle is adjustable.
 26. A method as defined in claim 20wherein said third stage is a chopper.
 27. A method as defined in claim20 wherein said regulated welding current has a power of over 2 KW. 28.A method as defined in claim 20 wherein said first stage has acontroller with a controller voltage and said method includes: (f)deriving said controller voltage from said first DC bus.
 29. A method asdefined in claim 28 wherein said third stage has a controller with acontroller voltage and said method includes: (g) deriving saidcontroller voltage of said third stage controller voltage from saidsecond DC bus.
 30. A method as defined in claim 20 wherein said thirdstage has a controller with a controller voltage and said methodincludes: (f) deriving said third stage controller voltage from saidsecond DC bus.
 31. The method of claim 20, wherein converting said ACpower supply signal to said first fixed DC bus voltage includesregulating and controlling said first fixed DC bus voltage to a voltagebetween 400 and 500 volts.
 32. The method of claim 31, whereinconverting the first fixed DC bus voltage provides that the second fixedDC bus voltage is about ¼ the voltage of the first fixed DC bus voltage.33. A power source for an electric arc welding process having a topologywith each stage of the topology defined by an input and an outputsignal, said power source comprising: an input stage having a rectifierand a DC to DC converter for converting an AC input signal to a firstfixed DC output signal; a second stage in the form of an unregulated DCto DC converter such that it is not adjusted by a real time feedbacksignal from the welding process, the second stage having an input in theform of a first DC bus for said first fixed DC signal and an output inthe form of a second DC bus for a second fixed DC output signal that isnot suitable for welding, electrically isolated from said first DCsignal and with a magnitude of a given ratio to said first fixed DCsignal; and a third stage regulated converter having said second fixedDC signal as an input signal to the third stage, said third stage beingregulated by a feedback signal from the welding process to convert saidsecond fixed DC signal to a current output signal suitable for welding,said third stage converting said second fixed DC signal separate anddistinct of said input and second stages.
 34. A power source as definedin claim 33 wherein said input stage DC to DC converter includes a powerfactor correcting converter.
 35. A power source as defined in claim 34wherein said power factor correcting converter is a buck+boostconverter.
 36. A power source as defined in claim 34 wherein said powerfactor correcting converter is an active converter.
 37. A power sourceas defined in claim 33 wherein said AC input is a three phase or singlephase input.
 38. A power source as defined in claim 33 wherein saidinput stage comprises a regulated converter stage having a controllerwith a control voltage and a voltage circuit to provide said controlvoltage from said first fixed DC signal.
 39. A power source as definedin claim 33 wherein said second stage is an inverter.
 40. A power sourceas defined in claim 39 wherein said inverter has switches with a highswitching frequency wherein said switches have a fixed duty cycle foroperating said switches.
 41. A power source as defined in claim 40wherein said duty cycle is fixed at about 100%.
 42. A power source asdefined in claim 40 wherein said duty cycle is adjustable.
 43. A powersource as defined in claim 40 wherein said duty cycle is controlled byphase shift of said switches.
 44. A power source as defined in claim 40wherein said high switching frequency is greater than about 18 kHz. 45.A power source as defined in claim 33 wherein said second stage has anisolation transformer with a primary winding and a second winding.
 46. Apower source as defined in claim 45 wherein said primary winding hassubstantially more turns than said secondary winding such that saidsecond fixed DC output signal is less than said first DC output signal.47. A power source as defined in claim 33 wherein said third stage is achopper.
 48. A power source as defined in claim 33 wherein said powersource has an output power capacity of at least 5 KW.
 49. The powersource of claim 33, wherein the first fixed DC output signal isregulated and controlled to a voltage between 400 and 500 volts.
 50. Thepower source of claim 49, wherein the second fixed DC output signal isabout ¼ the voltage of the first fixed DC output signal.
 51. A powersource for an electric arc welding process having a topology with eachstage of the topology defined by an input and an output signal, saidpower source comprising: a first power factor correcting input stage forconverting an AC input signal to a first regulated fixed DC bus voltageoutput signal on a first fixed DC bus; a second stage switching inverterhaving a primary side and a secondary side including a network ofswitches operated at a fixed duty cycle for converting said first fixedDC bus voltage as an input signal to the second stage to a second fixedDC bus voltage output signal not used for welding on a second fixed DCbus isolated from said first fixed DC bus, said second stage beingunregulated so as not to be adjusted by a real time feedback signal fromthe welding process and including: an isolating transformer having aprimary winding defining the primary side of said second stage and asecondary winding defining the secondary side of said second stage; saidsecond stage further including a plurality of capacitors including aprimary side capacitor and a secondary side capacitor, said network ofswitches operating across said primary side capacitor; and a regulatedthird stage converter having said second fixed DC bus voltage as aninput signal, said third stage converter converting said second fixed DCbus voltage into an output signal used for welding, said third stageconverter being regulated by feedback of a parameter from said electricarc welding process such that the third stage converts said second fixedDC bus voltage separate and distinct from said first and second stage.52. A power source as defined in claim 51 wherein said input stageincludes a rectifier and a power factor correcting converter said thirdstage includes a control circuit and a waveform generator for regulatingthe third stage converter to provide the regulated welding signal with afeedback parameter from said welding process.
 53. A power source asdefined in claim 52 wherein said power factor correcting converter is abuck+boost converter.
 54. A power source as defined in claim 52 whereinsaid power factor correcting converter is an active converter.
 55. Apower source as defined in claim 51 wherein said third stage is achopper.
 56. A power source as defined in claim 51 wherein said powersource has an output power capacity of at least 5 KW.
 57. The powersource of claim 51, wherein the first fixed DC bus voltage is regulatedand controlled to a voltage between 400 and 500 volts.
 58. The powersource of claim 57, wherein the second fixed DC bus voltage is about ¼the voltage of the first fixed DC bus voltage.
 59. A power source for anelectric arc welding process having a three stage topology with eachstage defined by an input and an output signal, said power sourcecomprising: a first stage having an AC input signal, a first fixed DCoutput signal, and a first stage controller with a voltage input and afirst control output for providing a first control signal to said firststage; a second stage having an input connected to said first fixed DCsignal for an input signal to the second stage, an output signal in theform of a second fixed DC output signal that is not used for welding andis electrically isolated from said first fixed DC signal, a switchingnetwork wherein said switches of said network have a fixed operatingduty cycle at a switching frequency of at least 100 kHZ as defined by asecond stage controller with a voltage input and a second control outputfor providing a second control signal to said second stage, said secondstage being unregulated such that said second stage is not adjusted by areal time feedback signal from said welding process; and a third stagehaving an input connected to said second fixed DC signal for an inputsignal to the third stage and a third stage controller with a voltageinput and a third control output for providing a regulated third controlsignal to said third stage for converting said second fixed DC signal toan output signal used for welding, the converting of the second fixed DCsignal being separate and distinct of said first and said stages. 60.The power source as defined in claim 59, wherein said second stage is anunregulated DC to DC isolation converter.
 61. The power source asdefined in claim 59, further comprising a first power supply providing afirst controller voltage to a voltage input of one of said controllers.62. The power source as defined in claim 61, wherein said first powersupply provides said first controller voltage to said voltage input ofsaid second controller.
 63. The power source as defined in claim 61,further comprising a second power supply providing a second controllervoltage to said voltage input of said second controller.
 64. The powersource as defined in claim 61, wherein said first power supply providessaid first controller voltage from said first fixed DC output signal.65. The power source as defined in claim 61, wherein said first powersupply provides said first controller voltage from said second fixed DCoutput signal.
 66. The power source as defined in claim 63, wherein saidsecond power supply provides said second controller voltage from saidsecond fixed DC output signal.
 67. The power source of claim 59, whereinsaid first fixed DC bus voltage is regulated and controlled at a levelbetween 400 and 500 volts.
 68. The power source of claim 67, wherein thesecond fixed DC bus voltage is about ¼ the voltage of the first fixed DCbus voltage.
 69. A method of MIG welding including: (a) advancing awelding wire toward a workpiece; (b) generating a welding output signalwith a three stage power source having each stage defined by an inputsignal and an output signal, the three stage power source including aregulated input stage with an AC input signal, a first fixed DC outputsignal and a center stage in the form of an unregulated DC-DC convertersuch that said center stage is not adjusted by a real time feedback ofthe welding output signal, the center stage having said first fixed DCsignal as an input signal with a second fixed DC output signal isolatedfrom and less than said first fixed DC output signal, said second fixedDC output signal not used for welding; and (c) creating said weldingoutput signal from said second fixed DC signal by regulating an outputstage driven by said second fixed DC signal as an input signal to saidoutput stage, said output stage being regulated by said welding outputsignal to convert said second fixed DC signal to said welding outputsignal which is used for welding, said output stage converting saidsecond fixed DC signal separate and distinct of said input and centerstages.
 70. A method as defined in claim 69 wherein said center stage isan unregulated isolation full bridge inverter.
 71. A method as definedin claim 69 wherein said welding output signal is a DC signal.
 72. Amethod as defined in claim 69 including: (d) providing a granular fluxaround said welding wire and on said workpiece.
 73. A method as definedin claim 69 wherein said welding wire is a flux cored electrode.
 74. Themethod of claim 69, wherein the first fixed DC output signal of theregulated input stage is controlled to a level between 400 and 500volts.
 75. The method of claim 74, wherein the second fixed DC outputsignal of the unregulated DC-DC converter is ¼ the voltage of the firstfixed DC bus.
 76. A power source for an electric arc welding processhaving a topology with each stage of the topology defined by an inputand an output signal, said power source comprising: an input stagehaving a rectifier and a preregulated power factor correcting converterfor converting an AC input signal into a first fixed DC output signalhaving a first magnitude; a second stage in the form of an unregulatedDC to DC converter such that said second stage is not adjusted by a realtime feedback signal from the welding process, said second stage havingan input connected to said first fixed DC signal, the second stageincluding: a controller; a network of switches switched at a frequencyof at least 100 kHz with a given duty cycle by said controller so as toconvert said first fixed DC output signal at said second stage inputinto a first internal AC signal, said duty cycle being fixed duringoperation of said power source; an isolation transformer with a primarywinding and a secondary winding for creating a second internal AC signaldifferent than the first AC signal; and a rectifier to convert saidsecond internal AC signal into a second fixed DC output signal having asecond magnitude related to a turn ratio defined by the primary andsecondary windings such that the second magnitude is less than the firstmagnitude and not used for welding; and a third stage having said secondfixed DC signal as an input signal to the third stage to convert saidsecond fixed DC signal to an output signal used for welding, said thirdstage being regulated by a feedback signal from said process such thatsaid third stage converts said second fixed DC signal separate anddistinct of said input and said second stages.